The field of this invention relates to a method of using a universal diagnostic antigen derived from pseudorabies (PR) virus or prepared by recombinant DNA techniques. More specifically, the invention relates to the use of non-enveloped structural proteins or nucleocapsid proteins as diagnostic antigens, which are complementary to the antigen of a PR enveloped subunit derived vaccine, thereby permitting PR virus carriers to be distinguished from vaccinated swine. For convenience of reference, the abbreviation "PR" is used herein to mean "pseudorabies38 .
Swine found positive in the standard assay procedure for the presence of serum antibodies to PR virus, such as the enzyme-linked immunosorbant assay (ELISA) are assumed to be actual or potential carriers of the virus. However, positive reactions may be due to recovery from natural infection and/or vaccine immunization or a combination of both, and the swine may not be carriers of the virus. Consequently, positive reactions for pigs may or may not indicate a carrier of the virus. False negative reactions are possible. However, negative pigs are considered non-carriers with the understanding that false negatives do occur occasionally.
Subunit vaccines for pseudorabies have been developed and they are prepared to contain less than the full complement of antigens from the virus and, specifically, only the glycoprotein antigens of the virus. U.S. Pat. Nos. 4,470,967 and 4,493,825 describe subunit vaccines for PR containing glycoprotein antigens. Subunit vaccines consisting of one or more viral enveloped glycoproteins should reduce losses due to clinical disease and reduce the spread of virus within and between swine herds. Further, subunit vaccinated pigs unlike pigs vaccinated with traditional vaccines can be certified free of PR by testing for antibody to viral components that are not part of a subunit vaccine. It is the ability to identify virus infected subunit vaccinated pigs that make the subunit vaccine useful in control programs.
Since not all subunit vaccines will consist of the same viral glycoprotein, veterinary diagnostic laboratories need to have access to accurate vaccination records of individual animals and also to have the capacity to test for antibodies to several different diagnostic antigens. These requirements will increase the cost of control programs but can be avoided with the use of a universal diagnostic antigen. Pseudorabies virus non-enveloped structural proteins or, more specifically, nucleocapsid proteins are logical candidates for a universal diagnostic antigen because they have not been shown to play a role in protective immunity and will most likely not be a component of subunit vaccines.